Various transparent substrate materials such as plastics or glass for use in display screens of display devices such as picture tubes, liquid crystal displays, etc., window materials for clean rooms, and packaging materials for electronic parts, or films for use in overhead displays or photographs usually belong to insulating materials and thus are liable to be electrostatically charged. Accordingly, dust or fine powder is liable to gather around substrate surfaces, sometimes resulting in glitch of electronic appliances, etc. Influences of electromagnetic waves generated from personal computers and television screens on human bodies have been a current problem.
To provide antistatic protection or electromagnetic wave shielding, it is the usual practice to apply a coating agent or a paint containing electroconductive materials to substrates or mold a mixture of substrate materials and electroconductive materials. Metal particles are usually used as such electroconductive materials. Particularly, metal particles having average particles sizes of approximately 1-100 nm, the called colloidal metal particles, have a property capable of transmitting visible light, and thus are suitable for use in the transparent substrate materials. They are the most suitable electroconductive materials for electromagnetic wave shielding requiring a high electroconductivity. The colloidal metal particles are usually used in a colloidal metal solution state, where the colloidal metal particles are dispersed in a dispersion medium. However, the colloidal metal particles have a large surface energy due to very small particle sizes, and thus are liable to aggregate and hard to disperse stably in such dispersion media as water, organic solvents, etc. For the desired stable dispersion, it is known to protect the surfaces of the colloidal metal particles with a stabilizer called “protective colloid” before the dispersion.
For example, M Carey Lea: American Journal of Science, Vol. 37, pp. 476-491, 1889, discloses a method for preparing a colloidal metal solution by adding citric acid or its salt to an aqueous solution of a metal salt as a protective colloid and then a reducing agent such as ferrous ions, etc. thereto, followed by desalting and concentration. The method requires a large quantity of the protective colloid for stabilizion of the dispersion of colloid metal particles, resulting in another problem of lowering of the electroconductivity of colloidal metal particles. Furthermore, the stabilization of the dispersion by a protective colloid makes it hard to reaggregate the colloidal metal particles, and removal of salts from the colloidal metal solution and concentration of the colloidal metal particles need such operations as centrifuging, ultrafiltration, deionization, etc., requiring so large-scale apparatuses as to make the mass production unsuitable.
JP-A-10-195505 discloses a method for preparing colloidal metal particulate powders with thiol-protected surfaces by subjecting a solution containing a metal salt and an amine to reduction reaction, followed by addition of a thiol to the solution. The method is based on concentration of a solution containing colloidal metal particles in a rotary evaporator to separate the colloidal metal particles as powders and thus problems as to the desalting and the productivity have not been solved yet by the method. That is, it is hard to prepare a colloidal metal solution in good dispersion state.